Lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires

Growing high-quality core-shell heterostructure nanowires is still challenging due to the lattice mismatch issue at the radial interface. Herein, a versatile strategy is exploited for the lattice-mismatch-free construction of III-V/chalcogenide core-shell heterostructure nanowires by simply utilizing the surfactant and amorphous natures of chalcogenide semiconductors. Specifically, a variety of III-V/chalcogenide core-shell heterostructure nanowires are successfully constructed with controlled shell thicknesses, compositions, and smooth surfaces. Due to the conformal properties of obtained heterostructure nanowires, the wavelength-dependent bi-directional photoresponse and visible light-assisted infrared photodetection are realized in the type-I GaSb/GeS core-shell heterostructure nanowires. Also, the enhanced infrared photodetection is found in the type-II InGaAs/GeS core-shell heterostructure nanowires compared with the pristine InGaAs nanowires, in which both responsivity and detectivity are improved by more than 2 orders of magnitude. Evidently, this work paves the way for the lattice-mismatch-free construction of core-shell heterostructure nanowires by chemical vapor deposition for next-generation high-performance nanowire optoelectronics.

Supplementary Fig. S5 Band alignments of as-constructed core-shell heterostructure NWs.a-e, UPS of GaSb/GeS core-shell heterostructure NWs, pristine GaSb NWs, pristine GaAs NWs, pristine InGaAs NWs, and GaSb/GeSe core-shell heterostructure NWs, respectively.The solid black lines mark the baselines and the tangents of the curves.The intersections of the tangents with the baselines indicate the edges of UPS.f-i, Type-I band alignment of GaSb/GeS heterostructure, Type-II band alignments of GaAs/GeS heterostructure, InGaAs/GeS heterostructure, and GaSb/GeSe heterostructure.
Ultraviolet photoelectron spectroscopy (UPS, ESCALAB XI+, ThermoFisher) is adopted to evaluate the band structures of as-constructed core-shell heterostructure NWs.The band structures are deduced from UPS of pristine GaSb NWs, GaSb/GeS core-shell heterostructure NWs, pristine GaAs NWs, pristine InGaAs NWs, and GaSb/GeSe core-shell heterostructure NWs.According to the linear intersection method, the valence band (EV) values of GaSb, GeS, GaAs, InGaAs, and GeSe are calculated as -4.73 eV, -5.27 eV, -5.81 eV, -5.18 eV, and -5.73 eV, respectively, by subtracting the width of He I UPS from the excitation energy of 21.21 eV.
Meanwhile, the work functions (EF) of GaSb, GeS, GaAs, InGaAs, and GeSe are calculated as -4.46 eV, -4.73 eV, -5.11 eV, -4.40 eV, and -5.12 eV, respectively, by adding Ev to the second electron cutoff energy.According to the bandgap values of GaSb (0.70 eV), GeS (1.50 eV), GaAs (1.40 eV), InGaAs (1.13 eV), and GeSe (0.97 eV) reported in the literatures [1][2][3][4] , the conduction band (Ec) values of GaSb, GeS, GaAs, InGaAs, and GeSe are calculated as -4.03 eV, -3.77 eV, -4.41 eV, -4.05 eV, and -4.76 eV, respectively.In this case, the band alignment diagrams of as-constructed GaSb/GeS, GaAs/GeS, InGaAs/GeS, and GaSb/GeSe core-shell heterostructure NWs are drawn approximated.Obviously, the heterostructures with expected heterostructure type, such as type-I of GaSb/GeS and type-II of GaAs/GeS, InGaAs/GeS and GaSb/GeSe are constructed successfully.Fig. S7 shows the electrical properties of as-constructed GaSb/GeS core-shell heterostructure NWs.From the I-V curves of Fig. S7a, the IDS of pristine GaSb NW MSM photodetector changes linearly with VDS, demonstrating the typical Ohmic contacts between Ni electrodes and GaSb NW.With a VDS of 3V, the IDS is 3.96 μ A. For the GaSb/GeS core-shell heterostructure NWs MSM photodetectors, the IDS are significantly reduced compared to that of pristine GaSb NW MSM photodetector.At the same time, the saturated IDS are observed under large bias voltages.The saturated IDS decrease from 304 to 217 and 12 nA for thin, medium, and thick shell NWs.In short, with the increase of shell thickness, the IDS of as-studied MSM photodetectors decrease, indicating the GaSb cores act as the main conductive channels (as shown in Fig. S7b), and the GeS shells limit the currents.
To further illustrate the current limiting mechanism, a band structure model is depicted in Fig. S7c for demonstrating the holes transport process at the interface of GaSb and GeS.Under a forward bias voltage, the holes inject from drain electrode to the main conductive channel of GaSb core and are collected at source electrode, as shown in Fig. S7b.The holes pass across the GeS shell during both the injection and collection processes.As shown in Fig. S7c, a forward bias heterojunction and a reverse bias heterojunction are formed at the injection and collection regions, which block the holes transport and result in the decreased currents compared to pristine GaSb NW MSM photodetector.At thermal equilibrium, the barrier of the forward bias heterojunction (due to the Fermi level coincident) is   ( is the electronic charge and   is electrostatic potential), and the barrier of the reverse bias heterojunction (due to the valence band offset) is ∆EV (EV is the valence band edge) 5,6 .Obviously, with the increase of bias voltage, the barrier of the forward heterojunction of   at injection region will decrease, which will benefit to the holes transport, resulting in the increase of IDS.At the same time, the barrier of the reverse heterojunction at collection region keeps ∆EV.In this case, the saturated IDS is observed at a large bias voltage.It is worth mentioning that in addition to the classical thermionic emission, tunneling or defect-assisted tunneling also plays role on the holes injection and collection processes 7,8 .With a thin shell, the barrier is possible thin and low, benefiting to the holes tunneling process.As a result, with the increase of shell thickness, the IDS decreases.Supplementary Fig. S8 Broad Based on the mechanism of visible light-assisted behavior, both 520 nm and 635 nm lights also can improve the infrared photodetection performance of photodetector fabricated by GaSb/GeS core-shell heterostructure NWs with an appropriate shell thickness.As shown in Fig. S9a, with a medium shell, when assisted light of 520 nm is on, the dark current of the infrared photodetector decreases from 269 nA to 187 nA.At the same time, the photocurrent increases from 10 nA to 27 nA.When auxiliary light of 635 nm is on (Fig. S9b), the dark current of the infrared photodetector decreases from 266 nA to 216 nA, and the photocurrent increases from 11 nA to 22 nA.For photodetector with a thick shell, with the assistance of visible lights of 520 The ultra-thin GeS shell also grows on the surfaces of GaSb NWs for studying the passivation effect.From the HRTEM image of Fig. S11a, with a growth time of 1 s, the shell of asconstructed GaSb/GeS core-shell heterostructure NWs is around 2 nm.As shown in the EDS mapping images of Fig. S11b, Ga and Sb dominate the NW core.At the same time, Ge and S dominate the shell.This finding is in line with the result of GaSb/GeS core-shell heterostructure NW with a thicker shell, as shown in Fig. 1.From the broad-spectrum photodetection behavior of Fig. S11c, it is found that GaSb/GeS core-shell heterostructure NW exhibits larger photocurrent compared to that of pristine GaSb NW, which is attributed to the surface passivation effect of ultra-thin GeS shell.At the same time, due to the negative photoresponse caused by the GeS shell, a reduced photocurrent is also observed at the near ultraviolet waveband of 405 nm.In short, the amorphous chalcogenide shells not only overcome the lattice mismatch, but also passivate the surface charge trappings of III-V NWs.c, d, Broad-spectrum photodetection behaviors of pristine GaSb NWs and GaSb/Al2O3 core-shell NWs with 2 nm and 5 nm Al2O3 shell, respectively.The laser intensities from 405 nm to 785 nm and 850 nm to 1550 nm are 0.05 mW•mm -2 and 6.00 mW•mm -2 , respectively.e, f, Infrared photodetection behaviors of GaSb/Al2O3 core-shell NWs with 2 nm and 5 nm Al2O3 shells under the illuminations of visible light, respectively.The laser intensities of 405 nm and 1550 nm are 0.05 mW•mm -2 and 5.00 mW•mm -2 , respectively.
Beyond amorphous GeS, the larger bandgap Al2O3 is also attempted to passivate the surface charge trappings of GaSb NWs, as shown in Fig. S12.The Al2O3 shells grow on the surfaces of GaSb NWs by atomic layer deposition method.The HRTEM image and EDS mapping images of Fig. S12a-b show that GaSb/Al2O3 core-shell heterostructure NW with a shell of 5 nm is successfully constructed.Ga and Sb dominate the NW core.On the other hand, Al and O dominate the shell.From the broad-spectrum photodetection behaviors of Fig. S12c-d, it is found that GaSb NWs with the Al2O3 shells of 2 nm and 5 nm both exhibit larger photocurrents compared to the pristine GaSb NW, which is attributed to the surface passivation effect of Al2O3 shells.Furthermore, the visible light-assisted infrared photodetection behaviors are also studied in Fig. S12e-f.When the visible light is on, the infrared photodetection currents can be distinguished hardly.This result can be attributed to the fact that a large number of carriers generated by visible light act as background carriers for infrared photodetection, which leads to the serious recombination of photogenerated carriers (generated by 1500 nm laser) 9,10 .The results show that the epitaxial larger bandgap Al2O3 shells can passivate the surface charge trappings of GaSb NWs effectively.Compared to the as-constructed III-V/chalcogenide coreshell heterostructure NWs, wavelength-dependent bi-directional photodetection behavior, visible light-assisted infrared photodetection behavior, and faster response times are not observed in GaSb/Al2O3 core-shell heterostructure NWs.Supplementary Fig. S13 Stability of as-fabricated GaSb/GeS core-shell heterostructure NW photodetector.a, Broad-spectrum photodetection behavior of GaSb/GeS core-shell heterostructure NW after being stored in an atmospheric environment for 10 days and 30 days.The laser intensities from 405 nm to 785 nm and from 850 nm to 1550 nm are 0.05 mW•mm -2 and 6.00 mW•mm -2 , respectively.b, Visible light-assisted infrared photodetection performance of GaSb/GeS core-shell heterostructure NW after being stored in an atmospheric environment for 30 days.
As presented in Fig. S13a, the as-fabricated photodetector still exhibits stable wavelengthdependent bi-directional photodetection behavior, which displays a negative photoresponse in the wavelength of 405-785 nm and a positive photoresponse in the wavelength of 850-1550 nm after being stored in an atmospheric environment for 10 days and 30 days.The photocurrent attenuations are less than 10% and 20% for 10 days and 30 days, respectively.As shown in Fig. S13b, the as-fabricated photodetector also shows the visible light-assisted infrared photodetection behavior after being stored in an atmospheric environment for 30 days.It is found that when the assisted light of 405 nm is on, the dark current of the infrared photodetector is significantly suppressed, and the photocurrent increases obviously.In short, benefitting the as-constructed core-shell nanostructure, the as-fabricated photodetector exhibits stable, repeatable, and robust photodetection performance, which promises the application in further optoelectronic devices.
As shown in Fig. S16, with the bandgap of 1.13 eV for the InGaAs core, both the InGaAs core and GeS shell can absorb the visible light (< 785 nm) and generate electron-hole pairs (processes I and I').Under the illumination of visible light, the photogenerated electrons in GeS would transfer to the InGaAs core, driven by the built-in electric field of the band offset of the conduction band (EC) (process II).In contrast, the photogenerated holes are trapped in the shell (process II').The injected electrons would then increase the majority carrier (electron) concentration of the n-type InGaAs core.In the case of the InGaAs core, the photogenerated holes transfer to the GeS shell, which is induced by the built-in electric field of the band offset of the valence band (EV) (process III).The effective separation of photogenerated electron-hole pairs leads to the significant photocurrent improvement of the InGaAs/GeS core-shell heterostructure NW in the visible waveband (Fig. 5a).Under the illumination of near-infrared light, only the InGaAs core absorbs the light, generating the electron-hole pairs (process I').
Owing to the efficient spatial separation of photogenerated carriers driven by the built-in electric field, the lifetime of photogenerated carriers in the InGaAs/GeS core-shell heterostructure NW is much longer than that in the pristine InGaAs NW, resulting in the improvement of photodetection performance in the near-infrared waveband (Figs.5a&b).In addition, the built-in electric field of the heterojunction also drives the charges to passivate the defects at the interface, improving the response speed (Fig. 5c) 14 .
Supplementary Fig. S17 The temporal photoresponse characteristics of the array imaging unit under the illumination of 850 nm laser.

Supplementary
Fig. S6 Construction of the GaAs/GeS, InGaAs/GeS, and GaSb/GeSe core-shell heterostructure NWs.a,b, Diameter, shell thickness statistics and XRD patterns of GaAs/GeS coreshell heterostructure NWs, respectively.c,d, Diameter, shell thickness statistics and XRD patterns of InGaAs/GeS core-shell heterostructure NWs, respectively.e,f, Diameter, shell thickness statistics, and XRD patterns of GaSb/GeSe core-shell heterostructure NWs, respectively.The diameter and shell thickness statistics are reproduced on 25 samples and the error bar represents the standard deviation.Supplementary Fig. S7 Electrical properties of GaSb/GeS core-shell heterostructure NWs.a, I-V curves of the pristine GaSb NW and GaSb/GeS core-shell heterostructure NWs with different shell thicknesses.b, Schematic of NW MSM photodetector.c, Holes transport in the GaSb/GeS core-shell heterostructure NW.
-spectrum photodetection performance of pristine GaSb NWs.Supplementary Fig. S9 Visible light-assisted infrared photodetection performance of GaSb/GeS core-shell heterostructure NWs.a,b, The visible light-assisted infrared photodetection performance of NWs with a medium shell, where 520 nm and 635 nm light act as assisted light, respectively.c,d, The visible light-assisted infrared photodetection performance of NWs with a thick shell, where 520 nm and 635 nm light act as assisted light, respectively.
Supplementary Fig. S12 Construction of GaSb/Al2O3 core-shell NWs and their photodetection behaviors.a, HRTEM image of GaSb/Al2O3 core-shell NW. b, EDS elemental mapping images of Ga, Sb, Al, O.All the scale bars are 20 nm.